Particle contamination from vacuum equipment in VLSI processing is estimated to be responsible for 40% of the total yield loss. As a consequence, it is important to control particle occurrence through use of standard techniques such as statistical process control. This, in turn, requires sensitive real-time particle sensors that function reliably in the environment of the process equipment to report particle occurrence. Since statistics for particle occurrence become more reliable as more particles are counted, sensors are preferably designed to detect as many particles as possible. In addition, since the frequency of particle occurrence often increases with decreasing particle size, sensors are also preferably designed to detect as small particles as possible.
Laser based particle sensors employ the principle that a particle passing through an intense laser beam will scatter light to a photodetector which then generates a measurable signal.
U.S. Pat. No. 4,739,177 by Peter Borden describes a particle sensor in which a laser beam is repeatedly reflected back and forth between mirrors This sensor is designed to function regardless of the atmospheric pressure, and is thus appropriate for use in vacuum systems. U.S. Pat. No. 4,804,853 by Peter Borden, Laslo Szalai and Jon Munson describes a particle sensor that uses a single laser beam and photocells mounted on the sensor body parallel to the beam. This sensor will also function regardless of the local pressure and can be used in vacuum systems. Neither of these patents, however, discloses specific means for shaping the transverse cross-section of the laser beam to provide the advantages discussed below.
The sensors described in the two patents cited above use a body made from a single piece of precision machined stainless steel or aluminum. Even with the relatively few optical elements used in these sensors, such a body is expensive to manufacture because of the large number of machining operations required. In addition, making changes to the optical system to tailor the sensor to various applications is difficult and requires re-design of the body. Thus, these earlier designs suffer disadvantages in manufacturing cost and design flexibility.